Prior art self-propelled carriages having caster wheels provided to the periphery of steering/driving wheels and configured so that the direction of the self-propelled carriage changes in accordance with changes in a steering direction of the steering/driving wheels have been disclosed in, e.g., Japanese Patent Laid-Open Publication No. 7-257387. A self-propelled carriage shall be described with reference to FIGS. 6, 7A, and 7B hereof.
FIG. 6 is a diagram used to describe a basic configuration of the prior art technology. In a self-propelled carriage 100 (also referred to below simply as a “carriage”), shafts 102, 102 are mounted on a carriage frame 101 in a longitudinal direction; sliding guides 103, 103 and an intermediate plate 104 are mounted on the shafts 102, 102 while allowed to be raised and lowered; the intermediate plate 104 is pressed down by springs 105, 105; a raising and lowering servo motor 106 is provided to a center of the intermediate plate 104; a ball screw 107 is raised and lowered by the servo motor 106; a driving section 112 is connected to a lower end of the ball screw 107; a driving wheel 111 is provided to the driving section 112; sliding guides 113 are mounted on four corners of the carriage frame 101; and caster wheels 120 are mounted on the sliding guides 113 via shafts 114 and springs 115 while allowed to be raised and lowered.
FIGS. 7A and 7B are a plan view and an operational diagram of the caster wheels shown in FIG. 6.
In FIG. 7A, the caster wheels 120 are composed of a stand 121, a holder 122 turnably mounted on the stand 121, and a wheel 123 rotatably mounted on the holder 122. Point A indicates a turning center of the holder 122, and point Y indicates a point of contact between the wheel 123 and a path surface.
A description shall be provided of a change when the carriage traveling toward F stops and starts to travel toward Re.
A force in a leftward direction as viewed in the drawing (toward Re) acts upon the stand 121. The wheels 123 start to rotate, but are casters and therefore tend to turn about the point A. However, a force of friction with the path surface is created at point Y, and the wheels 123 do not move upward or downward as viewed in the drawing. Instead, the stand 121 starts to turn as indicated by a trajectory M.
In principle, the turning does not occur when the point A and point Y are on a traveling line. This is because the wheels 123 may rotate while being pushed, but start to turn when the point A departs even slightly from the traveling line. The load-carrying platform abruptly starts to move toward a direction perpendicular to the traveling line at an early stage of turning so as to noticeably depart from the trajectory M. This causes the load-carrying platform to vibrate and undergo lateral wobbling.
In FIG. 7B, the semicircular trajectory M extends in front of the wheels 123 while being formed so that the stand 121 moves from point A to point B and then to point C. If the stand 121 is disposed to the front and the wheels 123 are disposed to the rear, the caster wheels 120 will then shake in a vertical direction as viewed in the drawing.
However, in an automated line, lateral wobbling and vibration in the load-carrying platform cannot be alleviated in order to forcibly perform a switchback operation on the traveling line. Large-scale lateral wobbling causes damage to components, e.g., when a precision part has been mounted on the load-carrying platform, and is therefore undesirable.
In other words, in the self-propelled carriage, it is desirable to reduce shaking or vibration in a width direction that occurs in the carriage in conjunction with the switchback operation.